Power storage devices are known to include secondary batteries, such as lithium ion batteries, nickel hydride batteries, and lead batteries, and electrochemical capacitors, such as electric double layer capacitors. Due to miniaturization of mobile devices, limitation of installation spaces, or the like, further miniaturization of power storage devices is sought, and thus attention is given to lithium ion batteries for their high energy density. Packaging materials for lithium ion batteries have conventionally been metal cans, however; multilayer films are increasingly used because they are lightweight, highly heat dissipating, and produced at low cost.
Such a lithium ion battery using the multilayer film as a packaging material uses a configuration in which battery contents (e.g., cathode, separator, anode, electrolyte solution) are covered with a packaging material including an aluminum foil layer to thereby prevent moisture from penetrating into the battery. A lithium ion battery using such a configuration is referred to as an aluminum laminated lithium ion battery.
Embossed lithium ion batteries, for example, are known as aluminum laminated lithium ion batteries. In such an embossed lithium ion battery, a recessed portion is formed at part of the packaging material by cold forming, battery contents are stored in the recessed portion, and the rest of the packaging material is folded back, followed by heat sealing the edge portions (e.g., see PTL 1). With such a lithium ion battery, a recess formed deeper by cold forming enables storage of more battery contents to achieve higher energy density.